>Here in slovenia, many worker unions were against the lowering the taxes on paychecks (higher net with same gross pay), because their political parties were against those, even if the workers themselves would benefit from those.

You can't lower taxes without some sort of tradeoff, which is usually a cut to services. I don't know the context here but it stands to reason that you are not giving the whole situation.

>We also have some areas (eg. a government rail company), where you cannot even get a job, if you don't get approved by one of the two unions, which mostly means that you have to pay off someone in the union to get an approval (for a very cozy job).

Corruption is going to exist without strict governance regardless of unions. I'm a unionized worker in the USA and no one needs to bribe their way into a job here. We have robust oversight and documentation of the hiring process.